The present invention relates to a sensor device for detecting wetting of a windshield.
German Patent No. 197 01 258 describes a sensor device that operates according to an optoelectronic principle for controlling wash/wipe systems for automotive windshields. The sensor device has multiple transmitters and at least one receiver for injecting and outputting via a coupling means a defined radiation into and out of the windshield whose wetting due to moisture or soiling is to be measured. The radiation is completely reflected at least once in the windshield on the (dry) surface of the windshield due to the injection angle and finally is outputted again to a receiver at a predetermined location. Total reflection is prevented by wetting of the surface of the windshield (air, water, ice, fog, soiling, etc.), resulting in wetting-dependent radiation losses due to output of a portion of the radiation from the original beam path, e.g., due to water droplets. The diminished radiation detected by the receiver is outputted as a sensor signal to a signal processing arrangement, where it is analyzed with regard to controlling a wash or wipe system for an automotive windshield, for example. A controller controls,the wiper motor in continuous or interval wiping mode as a function of the sensor signal mode analyzed.
According to German Patent No. 197 01 258 , the transmitters are arranged concentrically around the receiver on the coupling means or concentrically in sections, the coupling means having a circular or toroidal design. Thus, a circular base area is spanned by transmitters, receivers and coupling means. For this reason, the sensor, i.e., the sensor casing, is in the form of a round cylinder.
The sensitive area of the sensor is defined approximately by the sum of the measuring ranges of the windshield between a transmitter and the respective receiver, i.e., the one receiving the radiation. The measuring range is understood to be the range on the wettable side of the windshield within which the transmitter radiation is completely reflected in the absence of wetting, and therefore the transmitter radiation is outputted more or less in the measuring range because of wetting of the windshield.
One disadvantage here is the circular base area defined by the transmitters, receivers and coupling means and the round cylindrical sensor casing, leading to a relatively great extent of the sensor and its contact area on the windshield and therefore making the sensor noticeable and causing interference for the driver of the vehicle when it is mounted within the wiping area on the windshield, as is customary today.
Another disadvantage here is the small proportion of sensitive area relative to the base area of the sensor, i.e., its contact area. This is due to the fact that with a predetermined number of transmitters, e.g., eight, the non-sensitive areas between the transmitters arranged on the outer perimeter of the base area are large, usually larger than the sensitive areas.
This can be illustrated with an example. The circular base area spanned by the eight transmitters amounts to approx. 800 mm2 with a radius r of 16 mm. Transmitters and receivers are mounted 3 mm, for example, above the windshield. The injection angle is 45xc2x0 for a beam of light with a beam diameter of 5 mm. The eight individual sensitive areas between one sensor and one receiver are elliptical and yield a total sensitive area of approx. 55 mm2. The ratio of the total sensitive area to the base area of the sensor is therefore calculated as being at most 6%.
To increase the sensitive area, a greater number of transmitters could be used. However, this would mean an increase in the cost of the sensor without eliminating the disadvantages of a great extent or a large contact area of the sensor.
The sensor device according to the present invention has the advantage that the base area spanned by the transmitter and receiver is not a full circle but instead is at least one sector of a circle. For example, the base area is defined by a semicircle or by two opposite sectors with a central angle of about 90 degrees.
In addition, the ratio of sensitive area to base area is improved, in particular doubled to 12% with a semicircle having the same number of transmitters, e.g., eight, and receivers. In other words, the existing base area is utilized optimally to measure the wetting.
Due to the reduced base area, the outer dimensions of the sensor on the windshield and thus its contact area are likewise reduced. It is especially advantageous that due to the non-circular base area, the outer dimensions of the sensor casing can be further reduced by selecting a contact area that is not circular but instead is quadrilateral, in particular rectangular, so that the base area can be arranged in the contact area with optimal utilization of the latter. In addition, a rectangular contact area, i.e., casing, is less expensive to manufacture.
In a first embodiment, the receivers are arranged within a rectangular contact area of the sensor device so that the transmitters are arranged on two sectors, with each sector corresponding to approximately one fourth of a circle with their apexes facing one another to form the center of the circle. The receiver is at the common center of the sectors, with the transmitters arranged equidistant from the receiver on the outer edge of the sectors. One advantage here is that the width of the rectangular base area corresponds only approximately to the radius. The length of the rectangular base area is obtained from twice the radius.
Another advantageous embodiment is obtained when the transmitters and receivers are arranged in a semicircle within this rectangular contact area of the sensor device. The transmitters are on the outer edge of a semicircular sector, and the receiver is arranged at the center of this sector. The width of the base area here again advantageously corresponds approximately to the radius of the sector, with the length of the base area corresponding to the diameter of the sector.
A third embodiment has two sectors (quarter circles) which are rotated by 180xc2x0 relative to one another and are arranged side by side within the rectangular contact area. Two receivers are necessary for detecting the transmitter radiation.
Consequently, it is especially advantageous that the transmitters and receivers are arranged within the contact area of the sensor device, and the base area is selected with maximum utilization of the contact area, while at the same time the ratio of the sensitive area to the base area is also improved. Thus, an inexpensive sensor with small outside dimensions and at least uniform efficiency is obtained according to the present invention.
To further reduce the contact area of the sensor device, the distance from the transmitters to the receiver and thus the radius of the base area or the sector is minimized by the fact that the light emitted by the transmitters is subject to total reflection only on the outside of the windshield and then is output directly from the windshield to the receiver.